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SCIENCE 



[N. S. Vol. XXXVIII. No. 971 



do not understand me to say that these struc- 

 tures are not without a possible effect in the 

 organism. The mesonephros of mammals like- 

 wise represents an important adaptation of 

 the past, but Felix has once and again pointed 

 out that evidence of an excretory function is 

 lacking. But these illustrations will suffice. 

 As a record of the past history of the race, 

 they stand as a testimony to the very change 

 in adaptation that the organism has under- 

 gone with the progress of time and evolution. 

 As such they afford valuable clues and are 

 thus of taxonomic value. 



In the second group I include those adapta- 

 tions that exist or appear in the course of 

 development to meet the life conditions pecu- 

 liar to that period. These structures introduce 

 complexities in development. They are present 

 at one period of the life cycle and pass away 

 with changed conditions. Where traces of them 

 remain, they are like the vestigial structures 

 of the first group, a record of past adaptations, 

 but in the individual history and not primarily 

 that of the race. As an example, the Kiemen- 

 reste (gill-remnants) of frogs and toads stand 

 as a record of the early adaptations of the 

 frog in its larval period. No function can be 

 assigned them; they appear to have no past 

 history in the race. Again let me repeat I do 

 not say that they may not be without effect in 

 the organism. The most noteworthy instances 

 in this group of structures of interest to the 

 vertebrate embryologist are the fetal mem- 

 branes, structures developed out of the ani- 

 mal's body (essentially) mainly for the pro- 

 tection, nutrition and respiration of the indi- 

 vidual during the early period of its ontogeny 

 and subsequently discarded when no longer 

 needed. Since they are outside the body, they 

 are not continued as vestigial structures; only 

 insignificant folds and so-called ligaments 

 remain as more or less useless remnants. 



Such transient adaptations in the individual 

 life history have, of course, been evolved and 

 perfected in the evolution and share with 

 those of the first group a taxonomic value, but 

 with this difference: such adaptations to meet 

 very specific needs at a specific period in the 

 individual's life should, I believe, be used 



with caution. Let me give the two examples 

 that have impressed me most. 



In the development of the fetal membranes 

 of mammals a very marked variation in the 

 arrangement in the diflierent forms occurs. 

 In general the plan of development and rela- 

 tions appears to be broadly characteristic of 

 the different orders. In perhaps the majority 

 the amniotic cavity is formed by folding essen- 

 tially as in the reptiles and birds. In certaia 

 of the rodents, chiroptera, insectivora, and 

 probably primates, however, the amniotic cav- 

 ity appears precociously in the midst of the 

 ectoderm or trophoblast and only subsequently 

 do the typical structure and relations of the 

 amnion become established. An eminent 

 embryologist of Europe, Hubrecht, to whom 

 are due many of the facts of the early develop- 

 ment in these forms, concluded that this 

 method of formation of the amniotic cavity, 

 by dehiscence, is the primitive type and there- 

 fore decides in favor of an origin of the mam- 

 mals from amphibian-like forms. This 

 method of amnion formation appears, how- 

 ever, closely correlated with the method of 

 implantation of the ovum and placenta forma- 

 tion, and inasmuch as the type of placentation 

 represented is obviously the highest and most 

 direct the primitive character of amnion for- 

 mation by dehiscence may be seriously ques- 

 tioned. The uselessness of such a character 

 for taxonomic purposes is further illustrated 

 by the fact that in but one of the four groups 

 where it occurs is it apparently constant, but 

 amnion formation by folding is found as well 

 in certain of the forms. 



My second illustration of the questionable 

 character of such ontogenetic adaptations as 

 clues to genetic relations is the tadpole stage 

 of frogs and toads. The structural relations 

 of the larval organism depart in detail so 

 widely from the typical relations and are so 

 obviously correlated with the immediate life 

 conditions that one is justified, I believe, with 

 Spemann and Versluys in regarding the adult 

 as probably standing nearer the " ancestral 

 line." Founding broad genetic conclusions 

 from the conditions in the tadpole may be 

 done only with caution and reserve. The per- 



